[sdiy] Protection Device?

[Richie Burnett]

As Tom said, a series resistor is usually enough for digital inputs that have internal clamping diodes anyway for ESD protection. (I've seen examples in commercial circuits where 50/60Hz mains line voltage is used to clock PIC inputs via a couple of suitably large value current limiting resistors in series.)

And an RC filter is often sufficient for analogue inputs that have clamping diodes already present on the die. R limits clamping current, C provides low impedance to the ADC's sampling circuit, and RC provides some anti-aliasing filter action.

Just watch out for open-drain I/o pins that have one of the protection diodes missing. Was it RA4 on PICs? 

-Richie,

Sent from my Xperia SP on O2

---- Tom Wiltshire wrote ----

>The PIC does already have clamp diodes.
>
>My original schematic for the Druid VCDO chip specified external protection diodes as discussed. When we (me in collaboration with Frequency Central) actually released DIY boards for this project, we didn’t include the protection diodes and upped the series protection resistor to 4K7. So far (5+ years? I don’t remember any more), we’ve never had a report of a dead module from an over-voltage input. So op-amps are one way and do have a number of notable benefits like Olivier said - scaling, mixing, and the ability to add a specific offset being chief amongst them. But a “simple” solution can at least be robust too, if not quite as sophisticated.
>
>That said, I’d entirely recommend the op-amp solution for all the reasons given - scaling, mixing, an offset, and some filtering into the bargain. What’s not to like?!
>
>Tom 
>
>
>> On 19 Jun 2018, at 22:46, john slee <indigoid at oldcorollas.org> wrote:
>> 
>> Have you verified that your microcontroller doesn't already have internal clamp diodes on its input pins?
>> 
>> (some do... AVR I think, and maybe PIC? Though the supply voltage you mentioned elsewhere hints that you're not using AVR)
>> 
>> John
>> 
>> On 20 June 2018 at 05:52, Tim Ressel <timr at circuitabbey.com> wrote:
>> Hi,
>> 
>> I am forever duplicating a circuit for protecting processor inputs: a series current limiting resistor, a pair of schottky diodes for clamping to ground and pos rail, and sometimes a schmitt trigger buffer. Is there a single component that does this?
>> 
>> -- 
>> --Tim Ressel
>> Circuit Abbey
>> timr at circuitabbey.com
>> 
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